Combined frequency divider and detector



Oct. 28, 1947. M. G. CROSBY COMBINED FREQUENCY DIVIDER ND DETECTOR I Filed Jan. 19, 1945 Patented Oct. 2.8', 1947- Y 2,429,747 kyCOMBINED FREQUENCY DIVIDER AND DETEoToR Murray G. Crosby, Riverhead, N. Y., assigner to Radio Corporation of America, a corporation of Delaware Application January 19, 1945, Serial No. 573,594

(Cl. Z50-27) 12 Claims.

My present inventionrelates generally to detectors of angle modulated high frequency waves, and more `particularly to a novel method of, and means for, concurrently frequency dividing, discriminating and rectifying frequency modulated (FM) carrier waves.

In the past there have been proposed various systems for frequency dividing FM Waves for the purpose of concurrently reducing the carrier or center frequency of the vwaves and the deviation range proportionally. Frequency division networks have been disclosed in my U. S. Patent No. 2,064,106 granted December 15, 1936, and U. S. Patent No. 2,230,231 granted February 4, 1941. In the latter patent it is shown that the frequency divider may-employa locked-in oscillator. In the circuits of these patents, prior to demodulation of the .modulated carrier waves the latter are subjected to frequency division so as to reduce, the effective pass band of the modulated carrier wave in the same proportion as the carrier or center frequency is reduced. Such division is particularly advantageous inthe reception of frequency, or phase, modulated carrier' waves, since it has beenfound -to improve the signal to noise ratio at the outputl ofv the receiver and also to increase the selectivity with respect to adjacent channel interference (see Proc. I. R. E. for December 1944, A frequency-dividing locked-in oscillator frequency modulation receiver, G. L. Beers).

It is an important objectof-my presentl invention to provide a frequency divider which concurrently functions as a demodulator for angle modulated waves thereby eliminating the need for a. specialv demodulator network.

Another important object of my present in vention is to provide a locked-in oscillator system which functions concurrently as a discriminator detector thereby to provide economyr of construction.

In my U. S. Patent No. 2,269,417 granted January 6, 1942, I have disclosed and claimed an oscillator circuit generally comprising a pair `of electron discharge devices connected so that the cathode circuit of one device drives the cathode circuit of the other device. A tuned circuit is utilized in the plate circuit of one device and is coupled to the gridcircuit of the other device, the conjoint operation ofthe two devices having a negative trans-conductance characteristic.

It is another object of my present invention to employ an oscillator circuit'of the llast--Inentioned type as a locked-in oscillator.- with or without frequency division.

It is a more specic object of this invention to assign` an additional function to a frequency discriminator which may be the type disclosed and claimed by S. W. Seeley in his U. S. Patent No. 2,121,103 granted June 21, 1938, the additional function being that of a tank circuit for a lockedin oscillator feeding into the discriminator.

More specific objects of my invention are to employ a pair of oscillator circuits of the type shown in my aforesaid U. s. Patent No. 2,269,417 in push-pull relation; to utilize the resultant push-pull circuit as a balanced FM detector; and to cause said push-pull system to function as a frequency divider of the locked-in oscillator type.

Still other features will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing;

Fig. 1 showsan embodiment of the invention; and

Fig. 2 shows vector relations ideally existing at the discriminator at in-tune condition.

Referring now to Fig. 1 of the drawing, the circuit details of one embodiment of my invention are shown; it is to be understood that the lockedin oscillator system shown is adapted for use in any desired and suitable type of receiver of angle modulated high frequency waves. The generic term -angle modulated includes phase modulation (PM) frequency modulation (FM), orhybrid modulation possessing characteristics common to PM and FM. The receiving system is assumed to be a superheterodyne form of receiver, and the received Awaves are assumed to come from a transmitter radiating FM waves whose center frequency is in the to 50 ymegacycle (me.) band. Of course, a different type of receiver and any other frequency band suitable for FM or PM carrier wave transmission may be employed.

The numeral IA designates a suitable intermediate frequency (l. F.) amplifier network which is to be kemployed between a suitable converter, mixer or `first detector and the discriminator circuit. The operating I. F. value is preferably chosen from a range of 2 to 20 mc., and it may be assumed that an I. F. value of 5 mc. is employed. The network I may include one or more selective amplier stages each tuned to 5. mc. However, for operation under present FM broadcasting rstandards which provide for a frequency deviation up to kilocycles, each 551selector circuit, and this is true of all the signal selector circuits between the antenna and the output terminals of network I, is constructed to pass a band as wide as 200 kilocycles. Of course, such a pass band is substantially in excess of the maximum freqency swing of 150 kilocycles (kc.) permitted at the transmitter. At the FM transmitter the carrier wave is deviated in accordance with the modulating signal amplitude, the rate of deviation being a function of modulation frequel'lCy per Se.

The FM carrier waves usually are selectively amplified at 'carrier frequency and converted to I. F. without change in frequency deviation, and the I. F, energy is likewise selectively amplified at the I. F. value without change in frequency deviation range. However, for reasons stated above, itis desirable to present to the demodulator, or FM detector, signals whose-mean or center frequency and frequency swing or deviation have been reduced by a common factor. If the curve within rectangle I is ideally representative of the I. F. pass band, then the curve above transformer 2 is ideally representative of the desired pass bandV at the discriminator transformer 2. Comparing these curves it will be seen that whereas the pass band of network I is 200 kc. wide with a center frequency of F, the pass band of the discriminator transformer 2 is 200/n kc. while the center frequency is F/n kc. The factor n is an integer. If, for example, n is equal to 5, then the pass band width at the discriminator is 40 kc and the center frequency is 1 mc. The factor n may be unity or any larger number. In the former case there would be no frequency division.

According to my invention, the step of frequency division is secured by employing an oscillatory system which utilizes the teaching of my aforesaid U. S. Patent No. 2,269,417. However, in my present system the oscillatory system is caused to lock in with the FM signals at the output of network I whereby the frequency divided output at discriminator transformer 2 is faithfully controlled by the frequency deviation of the transmitted FM wave. The oscillatory system comprises a pair of twin triode tubes 3 and 4. Since the circuits and functions associated with the electrodes of tube 4 are the same as in the case of tube 3, there will be provided an explanation of the construction and operation of tube 3 and it will be understood that tube 4 duplicates the operation. Hence, similar numerals, but with prime designations, are used for tube 4 and its electrode circuits,

Tube 3 shows a pair of triodes, or electro-n discharge structures, T1 and T2 enclosed in the same envelope. While I have shown the use of a twin triode it is to be distinctly understood that my invention is not limited thereto, but that a pair of entirely separate and distinct tubes may be used. It should, further, be understood that I intend by the use of the term tubes to mean structures Comprising a cathode electrode, an anode electrode within an evacuated space, and means for influencing, or otherwise controlling, an electron stream between said electrodes regardless of whether the evacuated space is common to more than one electrode assembly or how it is maintained in an evacuated condition. Furthermore, the invention is not limited to the use of triode tubes, since pentode or beam tubes may be used. Different types of tubes such as the triode and a beam tube may be combined.

The cathodes I I and 2| of tube 3 are connected in common through the unbypassed resistor 5 to ground; in the same way cathodes Il and 2|' of tube 4 are connected by unbypassed resistor 5' to ground. The grid or control electrode I2 of triode T1 is connected through the direct current blocking condenser 3E! to one side of the resonant secondary circuit of transformer 2. The resonant secondary circuit consists of secondary coil 3| shunted by condenser 32. The primary circuit of the transformer consists of the primary coil 33 shunted by condenser 34. Each of the primary and secondary circuits of transformer 2 is tuned to the frequency F/n. In other words, if n equals 5, the primary and secondary circuits of transformer 2 are each tuned to the fifth subharmonic of the frequency F. The control electrode I2 of triode Ti is connected through direct current blocking condenser 30 to the opposite side of the secondary circuit 3|, 32.

Each of control electrodes I2 and I2' is connected to ground through a respective one of the direct current return resistors 4G and 40'. The respective anodes or plates 23 and 23 of triodes T2 and T'z are connected in parallel to a lead 50. Lead 53 divides into a direct current connection 5I to the upper end of primary coil-33 and a second direct connection 52 to the midpoint of the secondary coil 3|. The lower end of coil 33 is connected to a -I-B terminal of a suitable source of direct current thereby to apply a desired positive potential to the parallel-connected plates 23 and 23. The lower end of coil 33 is connected to ground for high frequency currents by condenser 60.

The signal input waves for the tubes 3 and 4 are derived from the output terminals of the I. F. amplier network I. One of these output terminals is the ground terminal, Whereas the other terminal is connected by lead 'I0 to the control grids 22 and 22 of triodes T2 and T'z. In other words, the respective input electrodes are connected in parallel to the high alternating potential output terminal of the I. F, amplifier network I. Accordingly, it will be appreciated that the potential of the control grids 22 and 22 is varied in accordance with the frequency Variations or swing of the FM signal output energy of the I. F. network I. The oscillations generated in the oscillatory system are locked in to follow the frequency variations at grids 22 and 22', but the oscillations generated and developed in the secondary circuit 3|, 32 are of a mean or center frequency and frequency deviation reduced respectively by a factor of n relative to the input signals.

Considering, first, the manner in which tube 3 and its associated circuit connections function to provide oscillations, the common cathode resistance 5 is utilized for cathodes II and 2| so that the output voltage which results from the impression of voltage on grid I2 is fed to the cathode circuit of triode T2, as well as to that of triode T1. Utilizing the cathode circuit of T1 as an output circuit results in inverse feedback on grid I2, and the Voltage applied to grid I2 appears unamplifled on its associated cathode II. Due to this high degree of inverse feedback the grid I2 may be swung over a wide range of voltage without drawing grid current so that the impedance presented to the tuned circuit 3|, 32 by the grid is that of a negatively biased or class A grid, which is very high.

Triodes T1 and T2 constitute a negative transconductance device, since the resulting transcon ductance between the inputgrid IZ and the output plate 23 is negative with respect to the usual single-triode transconductance. It is made negative in the present-case by the phase reversal produced -by the common cathode resistor drive. In order lto produce sustained oscillations the feedback voltage from plate 23 (also 23") is applied to secondary circuit 3|, 3.2 by means of lead 50 and the primary circuit .34, r33, the latter being magnetically coupled totuned-circuit 3|, 32. This feedback voltage .must .be fed back VVto the grid .22 of -triode T2 with la 180 phase reversal in order to :produce oscillations. This is readily accomplisheddue Yto the fact that the common cathode resistor leifects such reversal. A positive voltage on .the .grid :I2 tends `to cause more current to ow ithrough :the resistor ."5 and thereby makes` both `cathode and cathode .2| .more positive. The'ycathode :2| .becoming more positive is equivaient to grid 22 becoming more negative. Thus, azpositive voltage .changeon the grid I2 .is equivalentto anegative `voltagechange .ongrid 22, andr the required 180fphase reversal is thus obtained.

.The tube -4 `and `its associated circuits functionV in :the same way :to sustain oscillations .at the desired divided frequency A-F/n. "However, it :will be noted that the voltage Ep .across the primary circuit 34, `is .applied to the grids I2 and |.2 of the fdischarge devices T1 and Ti in push-pull relation `by. virtue .of the magnetic coupling between lcoils 38 and V3|. These induced voltages across secondary winding `3| are representedn in Fig. 2 bythe Vectors ES and .Es. `These voltages are equal fand :of v.opposite polarity.

At `the `same time, `and because of .the direct connection vofthe-,upper end of coil 33 to the midpoint of coil 3| thrQugh-leadsiEI.anda.52, the entire primary voltage -Ep is .appliedin .paralleL-i. e. in like phase, tto vthegrids I2 and I2. Hence, the vector representation Ep in Fig. ,2 is shown in phase fquadrature to .voltages .Esand yEs. `It will be understood that-whenthe I. lenergy has an instantaneous frequency vF-,the frequency divided FM energycat `transformer'2 has-afrequency F/n, and, therefore, a 90 phase shift occurs in the transfer of energy from the Aprimary circuit 34, 3-3 to the secondary circuit 3|, 3-2 by means of the inductivecoupling. `For instantaneous frequencies: deviating ,from the centerfrequencythe phase shift Vdeparts from phase quadrature;

rIfheresultantvoltages at Ygrids I2 and :|2 at frequency vFm areindicatedin Fig. 2 by vectors as Eg and Eg. lObviously at the center frequency ofthe frequency divided signal energy the Avoltages Egand Eig will befequal. However, should the instantaneous frequency offthe locked-in oscillations-.depart ,from the center l'frequency F/n, then the -vectors AEg-and-Eg will become unequal in accordance "with ythe usual and Well known manner of operation ofthe'Seeleytype discriminator. vSince the ,phase relations `between the voltageEp and eachoffvoltages Eg'a'nd Eg is less than 90, the discriminator phase lrelations will in no way prevent the conditions'suitable for sustaining oscillations. n

The action referredto in the preceding `paragraph -m-aybefurther explained v as follows. :Potentials conveyed by llead ,52 :to the Amidtap of secondary 3| of transformer 2 areimpressedin likephase on'grids l2 and I2 of -triodes T1 vand .'I"1 respectively. Such potentials result in like phase currents in parallel through thefrespective anode-cathodefspace discharge paths `23, V2| yand 2,3', 2|"0f triodes'zTzand Tz. Potentials transferred acrosstransformer 2 Ito secondary 3| are impressed in .opposite phaseon grids |-2 .and I2. However, as concerns the production of oscillations, '.it .may be considered :that .the 1in-phase potentials on .grids I2 and I2 .are the controlling ones. If vWe assume an instantaneous potential, positiv-cor negativaon grids I2 and l2 :bytransfer of energy to the midtap on secondary 3|, `the action of transformer 2, vif the generated frequency is at one -side ofthe mean oscillator frequency, may be to radd to the instantaneous Vpotential on grid -I2 and to subtract from that on grid I 2. When `the generated frequency changes to the other side of -the mean frequency, the action of transformer 2 vwill then be to add to the instantaneous 4potential `on grid -|2 and to subtract from that on grid SI2. The resulting effect is to cause an increase in the intensity Yof oscillation inthe tube receiving the adding effect from transformer -2 and to decrease the intensity in the tube receiving the subtracting effect,

The respective voltages 'Eg and Eg are rectified by devices T1 and Ti. The rectification occurs by virtue lof the respective grid leak circuits S, 40 and 30', 40. It Will be noted that plate l) is connected vto Va source of positive potential through a load resistor 'II which is bypassed for high frequency currents by condenser l2. In the same Way plate 'I0' is provided with a load resistor 1| also bypassed for high frequency currents by condenser 'I2'. These load resistors 1I .and 'II' respectively develop thereacross rectified voltages.

It will be obvious that these rectified voltage-s represent the frequency variations of the lockedin oscillations. They may be differentially `combined to feed `a single ended modulation amplifier, or they may be separately appliedto the input terminals of a push-pull modulation amplifier. At the instantaneous center frequency of the locked-in oscillations the respective rectified voltagesacross resistors 'II and 'I I are equal, and thediiferential combining thereof Will provide zero output since the rectified voltages are of like polarity. Zero output Will, also, be secured by feeding the voltages to a push-pull combining system. However, for departures from center or carrier frequency the voltages of the rectiflers Will be unequal, and, therefore, the differential output voltage will have a magnitude and polarity dependent on the amount and direction of deviation from center or carrier frequency. This is similar to the action in the known balanced diode detector system.

By properly choosing circuit constants and electrode voltages for the oscillator tubes the lockedin oscillator-system is provided with a substanti-ally constant current output regardless of signal amplitude variations at the input of network I. The locked-in oscillator maybe adjusted to have a wide` lock-in range by adjusting the band width of the band pass transformer 2 to have a wide band Width. Hence, the lock-in range may be controlled by adjustment of the effective band width of transformer 2.

While E have indicated and described a System for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by yno `means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In combination with a source of angle modulated carrier waves of a predetermined center frequency and deviation range, a. frequency divider locked-in oscillator comprising a pair of oscillator systems -having separate input elec- 7 trodes connected in parallel to said source, a common tank circuit for said oscillator systems electronically coupled to said input electrodes thereby to provide lock-in over said deviation range, said tank circuit being tuned to a predetermined subharmonic frequency of said center frequency, each of said oscillator systems including a respective output circuit and a respective control electrode therefor, said tank circuit being constructed as a phase shifter and providing from frequency divided, locked-in oscillations a pair of voltages Whose relative magnitude and phase are dependent on the amount and direction of deviation of the locked-in oscillations from said subharmonic frequency, and means for applying each of the pair of voltages to a respective one of said control electrodes thereby to provide a rectified voltage in each output circuit.

2. In combination with a source of angle modulated carrier waves of a predetermined center frequency and deviation range, a locked-in oscillator comprising a pair of oscillator systems having separate input electrodes connected in parallel to said Source, a common tank circuit for said oscillator systems electronically coupled to said input electrodes thereby to provide lo-ck-in over said deviation range, said tank circuit being tuned to a frequency having a predetermined re- 8 of one of the devices of each of said pairs of devices, a second resonant circuit coupled to said first resonant circuit, means for coupling to points of opposite polarity of said second resonant circuit respective grids of the second of said devices of each of said pairs, means for maintaining the respective anodes of said second clevices at relatively fixed positive potentials, means connected between the cathodes of each pairs of devices and a point of Zero reference potential for coupling the cathodes of each pair of devices together, means for varying the respective grids of said one devices with a frequency-Variable voltage, and said rst and second resonant circuits each being tuned to a desired subharmonic frequencyof saidfrequency-variable voltlation to said center frequency, each of said oscilt lator systems including a respective output circuit and a respective control electrode therefor,y

said tank circuit being constructed as a phase shifter and providing from locked-in oscillations a pair of voltages Whose relative magnitude and phase are dependent on the amount and direction of deviation of the locked-in oscillations from said tank circuit frequency, means for applying each o1" the pair of voltages to a respective one of said control electrodes thereby to provide a rectified voltage in each output circuit, and means for differentially combining the said rectified voltages for utilization.

3. In combination With a source of angle modulated carrier Waves of a predetermined center frequency, a locked-in oscillator comprising a pair of oscillator systems having separate input electrodes connected in parallel to said source, a common tank circuit for said oscillator systems electronically coupled to said input electrodes thereby to provide lock-in with said Waves, each of said oscillator systems including a respective output circuit and a respective control electrode therefor, said tank circuit being constructed as a phase shifter and providing from locked-in oscillations a pair of voltages whose relative magnitude and phase are dependent on the Yamount and direction of deviation of the locked-in oscillations, and means for applying each of the pair of voltages to a respective one of said control electrodes thereby to provide a rectified voltage in each output circuit.

4. In a system as defined in claim 3, said tank circuit comprising a primary circuit connected in parallel to respective anodes associated with said input electrode, a secondary circuit inductively coupled to said primary circuit, a direct current connection from one side of the primary circuit to the midpoint of the secondary circuit, and said control electrodes being connected to opposite sides of said midpoint.

5. In a frequency dividing, locked-in oscillator system, a pair of oscillator circuits, each circuit comprising a pair of electron discharge devices each having an anode, cathode and a grid, arrst' resonant circuit coupled in parallel to like anodes age.

6. In a locked-in oscillator system, a pair of oscillator circuits, each circuit comprising a pair of electron discharge devices each having an anode, cathode and a grid, a iirst resonant circuit coupled in parallel to like anodes of one of the devices of each of said pairs of devices, a second resonant circuit coupled to said first resonant circuit, means for coupling to points of opposite polarity of said second resonant circuit respective grids of the second of said devices of each of said pairs, means for maintaining the respective anodes of said second devices at relatively fixed positive potentials, means connected between the cathodes of each pairs of devices and a point of zero reference potential for coupling the cathodes of each pair of devices together, means for varying the respective grids of said one devices With a frequency-variable voltage, separate means in circuit with each of said respective grids of the second of said devices for providing rectification, and an output load resistor in circuit With each of the anodes of said second devices for developing rectified voltage thereacrossl 7. In combination with a source of frequency modulated carrier Waves of a predetermined carrier frequency and maximum deviation range, a frequency divider locked-in oscillator comprising a pair of oscillator systems having separate input electrodes connected in parallel to said source, a common tank circuit for said oscillator systems electronically coupled to said input electrodes thereby to provide flock-in with said deviation, said tank circuit being tuned to a predetermined subharrnonic frequency of said carrier frequency, each of said oscillator systems including a respective output circuit and a respective control electrode therefor, said tank circuit being constructed as a phase shifter and providing from frequency divided, locked-in oscillations a pair of voltages Whose relative magnitude and phase are dependent on the amount and direction of deviation of the locked-in oscillations from said subharmonic frequency.

8. In combination With a source of angle modulated carrier waves of a predetermined center frequency and maximum deviation range, a frequency divider locked-in oscillator comprising a pair of oscillator systems having separate input electrodes connected in parallel to said source, a common tank circuit for said oscillator systems electronically coupled to said input electrodes thereby to provide lock-in with said deviation, said tank circuit being tuned to a predetermined subharmonic frequency of said center frequency, each of said oscillator systems including a respective output circuit and a respective control electrode therefor, said tank circuit being constructed as a phase shifter and providing from frequency divided, locked-in oscillations a pair of voltages whose relative magnitude and phase are dependent on the amount and direction of deviation of the locked-in oscillations from said subharmonic frequency, and means for applying each of the pair of voltages to a respective one of said control electrodes thereby to provide a rectified voltage in each output circuit, and means for differentially combining the said rectified voltages.

9. In combination with a source of angle modulated carrier waves of a predetermined center frequency and maximum deviation range, a frequency dividel1 locked-in oscillator comprising a pair of oscillator systems having separate input electrodes connected in parallel to said source, a common tank circuit for said oscillator systems electronically coupled to said input elec-` trodes thereby to provide lock-in with said deviation said tank circuit being tuned to a predetermined subharmonic frequency of said center frequency, each of said oscillator systems including a respective output circuit and a respective control electrode therefor, said tank circuit being constructed as a phase shifter and providing from frequency divided, locked-in oscillations a pair of voltages whose relative magnitude and phase are dependent on the amount and direction of deviation of the locked-in oscillations from said subharmonic frequency, means for applying each of the pair of voltages to a respective one of Said control electrodes thereby to provide rectified voltage in each output circuit each of said control electrodes including in circuit therewith a respective condenser-resistor network to provide grid rectification, said tank circuit comprising a primary circuit connected in parallel to respective anodes associated with said input electrode, a secondary circuit inductively coupled to said primary circuit, a direct current connection from one side of the primary circuit to the midpoint ofthe secondary circuit, and said control electrodes being connected to opposite sides of said midpoint.

10. In a frequency dividing, locked-in oscillator system, a pair of oscillator circuits, each circuit comprising a' pair of electron discharge devices each having an anode, cathode anda grid, a first resonant circuit coupled in parallel to like anodes of one of the devices of each of said pairs of devices, a second resonant circuit coupled to said first resonant circuit, means for coupling to points of opposite polarity of said second resonant circuit respective grids of the second of said devices of each of said pairs, means for maintaining the respective anodes of said second devices at re1- atively fixed positive potentials, means connected between the cathodes of each pairs of devices and a point of zero reference potential for coupling the cathodes of each pair of devices t- 10 gether, means for varying the respective grids of said one devices with a frequency-variable voltage, and said first and second resonant circuits eacn being tuned to a desired subharmonic frequency of said frequency-variable voltage.

11. In a frequency dividing, locked-in oscillator system, a pair of oscillator circuits, each between the cathodes of each pairs of triodes and a point of zero reference potential for coupling the cathodes of each pair of devices together,l

means for varying the respective grids of said one triodes with a frequency-variable voltage, and said first and second resonant circuits each being tuned to a desired subharmonic frequency of said frequency-variable voltage, separate means in circuit with each of said respective grids of the second of said triodes for providing grid rectiication, and an output load resistor in circuit with each of the anodes of said second triodes for developing rectified voltage thereacross.

l2. In combination with a source of angle modulated carrier waves of a predetermined center frequency and deviation range, a locked-in oscillator comprising a pair of oscillator systems having separate input electrodes connected in parallel to said source, a common tank circuit for said oscillator systems electronically coupled to said input electrodes thereby to provide lockin over said deviation range, each of said oscillator systems including a respective output circuit and a respective control electrode therefor, said tank circuit'being constructed as a phase shifter and providing from locked-in oscillations a pair of voltages whose relative magnitude and phase are dependent on the amount and direction of deviation of the locked-in oscillations, and the lock-in range of said oscillator being dependent upon the band width of said tank circuit.

MURRAY G. CROSBY.

REFERENCES CITED The following references are of record in the file `of this patent: 

